Embodiments of the invention relate in general to lighting devices which include light conversion materials and in particular to light conversion materials for use in lighting devices comprising light emitting diodes (LEDs), where the conversion materials include combinations of semiconductor nanoparticles and phosphors based on rare-earth elements for light conversion and light conditioning.
LEDs offer significant advantages over incandescent and fluorescent lamps with respect to their high energy efficiency and long lifetimes. LEDs are applicable in diverse applications including displays, automobile and signage lighting and domestic and street lighting. A LED can emit monochromatic light in different regions of the spectrum, depending on the inorganic semiconductor compound used to fabricate it. However, “white” light, which is required for a very large portion of the lighting industry, cannot be generated using a conventional LED. Current solutions of producing white light include the use of three or more LEDs with various colors (e.g. Red, Green and Blue or “RGB”), or the use of a color conversion layer of phosphor material (e.g. Cerium:YAG) to generate a broad white spectral emission from the ultraviolet (UV) or blue emission of a LED. However, such white light is almost always non-ideal and has in many cases undesired or unpleasant characteristics which may require improvement or correction.
For display purposes, it is important to have three or more basic colours with narrow FWHM spectrum emission, such as that obtained with LEDs (FWHM typically <30 nm). This allows large gamut coverage. “Gamut” is usually defined as: the range of chromaticities that can be obtained by mixing three colours. The solution of using three or more different LED is expensive and complicated for some applications. Therefore it is desirable to have a light source that provides large gamut coverage with one type of LED.
One method for utilizing LEDs to provide a broad-spectrum light source utilizes phosphors which convert the LED light to light having longer wavelengths in a broad spectrum. For example, a phosphor that emits light over a broad range of green wavelengths can be illuminated with blue from an LED that generates a narrow blue spectrum. The phosphor-generated green light is then used as a component of the white light source. By combining several phosphors, one can, in principle, create a broad-spectrum white light source provided the light conversion efficiencies of the phosphors are sufficiently high. More details can be found in “Status and prospects for phosphor-based white LED Packaging”, Z. Liu et al., Xiaobing Front. Optoelectron. China 2009, 2(2): 119-140.
Unfortunately, a lamp designer does not have an arbitrary set of phosphors from which to choose. There are a limited number of conventional phosphors containing rare-earth elements that have sufficient light conversion efficiencies. The emission spectrum of these phosphors is not easily changed. Furthermore, the spectra are less than ideal in that the light emitted as a function of wavelength is not constant. Hence, even by combining several phosphors, an optimum white light source is not obtained.
U.S. Pat. Nos. 7,102,152 7,495,383 and 7,318,651, which are incorporated herein by reference in their entirety, disclose devices and methods for emitting output light utilizing both quantum dots (QD) and non-quantum fluorescent materials to convert at least some of the original light emitted from a light source of the device to longer wavelength light to change the colour characteristics of the output light. QD have high QY and narrow emission spectrum with a central emission wavelength (CWL) tunable by size. Combining both QD and phosphor can enhance the light quality. QD additives can offer improvement, but they suffer from high self absorbance, i.e. they absorb light that is emitted when excited. This lowers their total energy efficiency as light convertors. Moreover and most significantly, the QD also re-absorbs the phosphor emission, which reduces the energetic efficiency and also shifts the output spectrum such that rational color planning is very difficult.
Moreover, in some applications cluster of close-packed QDs is desired. Close-packed QD clusters exhibit the phenomenon known as Fluorescence Resonant Energy Transfer (FRET), see e.g. Joseph R. Lakowicz, “Principles of Fluorescence Spectroscopy”, 2nd edition, Kluwer Academic/Plenum Publishers, New York, 1999, pp. 367-443. FRET occurs between a donor QD which emits at a shorter (e.g. bluer) wavelength relative to an acceptor QD positioned in close proximity and which emits at longer wavelength. There is a dipole-dipole interaction between the donor emission transition dipole moment and the acceptor absorption transition dipole moment. The efficiency of the FRET process depends on the spectral overlap of the absorption of the donor with the emission of the acceptor. The FRET distance between quantum dots is typically 10 nm or smaller. The efficiency of the FRET process is very sensitive to distance. FRET leads to colour change (red shift) and losses in the efficiency of light conversion. Hence, in prior work efforts were made to avoid clustering of QDs in light conversion materials.
Core/shell nanoparticles (NPs) are known. These are discrete nanoparticles characterized by a heterostructure in which a “core” of one type of material is covered by a “shell” of another material. In some cases, the shell is grown over the core which serves as a “seed”, the core/shell NP known then as a “seeded” NP or SNP. The term “seed” or “core” refers to the innermost semiconductor material contained in the heterostructure. FIGS. 1A-1C show schematic illustrations of known core/shell particles. FIG. 1A illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. FIG. 1B illustrates a rod shaped (“nanorod”) SNP (RSNP) which has a core located asymmetrically within an elongated shell. The term nanorod refers to a nanocrystal having a rod-like shape, i.e. a nanocrystal formed by extended growth along a first (“length”) axis of the crystal with very small dimensions maintained along the other two axes. A nanorod has a very small (typically less than 10 nm) diameter and a length which may range from about 6 nm to about 500 nm.
Typically the core has a nearly spherical shape. However, cores of various shapes such as pseudo-pyramid, cube-octahedron and others can be used. Typical core diameters range from about 1 nm to about 20 nm. FIG. 1C illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. The overall particle diameter is d2, much larger than the core diameter d1. The magnitude of d2 compared with d1 affects the optical absorbance of the core/shell NP.
As known, a SNP may include additional external shells which can provide better optical and chemical properties such as higher quantum yield (QY) and better durability. The combination may be tuned to provide emitting colors as required for the application. In a RSNP, the length of the first shell can range in general between 10 nm and 200 nm and in particular between 15 nm and 160 nm. The thicknesses of the first shell in the other two dimensions (radial axis of the rod shape) may range between 1 nm and 10 nm. The thickness of additional shells may range in general between 0.3 nm and 20 nm and in particular between 0.5 nm and 10 nm.
In view of the numerous deficiencies of known light conversion materials mentioned above, including known QD-conventional phosphor combinations there is a need for and it would be advantageous to nanoparticle-phosphor combinations and compositions of materials including such combinations which do not suffer from such deficiencies. In particular, there is a need for and it would be advantageous to have nanoparticle-phosphor combinations with small to negligible re-absorption of the phosphor emission by the nanoparticles and with small self-absorbance of the nanoparticles, thus leading to high conversion efficiencies and ultimate color gamut control. In addition, these combinations should have negligible FRET upon clustering and high-loading in the conversion materials.